Barium titanate-based semiconductive ceramic composition

ABSTRACT

A barium titanate-based semiconductive ceramic composition for facilitating miniaturization of thermistor devices by improving rush current resistance characteristics is provided. In the barium titanate-based semiconductive ceramic composition, a fraction of the Ba in BaTiO 3  as the major component is replaced with 1 to 25 mole percent of Ca, 1 to 30 mole percent of Sr, and 1 to 50 mole percent of Pb; and wherein to 100 mole percent of the major component, the semiconductivity-imparting agent is added in an amount of 0.2 to 1.0 mole percent as a converted element content, and the additive comprises manganese oxide in an amount of 0.01 to 0.10 mole percent as a converted Mn content, silica in an amount of 0.5 to 5 mole percent as a converted SiO 2  content, and magnesium oxide in an amount of 0.028 to 0.093 mole percent as a converted Mg content.

TECHNICAL FIELD

The present invention generally relates to semiconductive ceramic compositions, and more specifically to a barium titanate-based semiconductive ceramic composition.

BACKGROUND ART

The following conventional barium titanate-based semiconductive ceramic compositions are known. Japanese Patent Publication No. 62-43522 discloses a barium titanate-based semiconductive ceramic composition, which is substantially composed of BaTiO₃ or in which Pb is partly substituted for Ba, and which contains 0.00035 to 0.0072 percent by weight of magnesium when the weight of the composition is 100, for the purpose of increasing withstand voltage.

Japanese Patent Publication No. 63-28324 discloses a barium titanate-based semiconductive ceramic composition containing 30 to 95 mole percent of BaTiO₃ as the major component, 3 to 25 mole percent of CaTiO₃, 1 to 30 mole percent of SrTiO₃, and 1 to 50 mole percent of PbTiO₃, in which a fraction of Ba is replaced with Ca, Sr and Pb in order to improve withstand voltage and rush current resistance characteristics.

Furthermore, Japanese Patent Publication No. 62-58642 discloses a semiconductive ceramic composition having a rush current which is not large, and a positive resistance-versus-temperature property with a small change over time in an intermittent test, in which Ba in barium titanate is replaced with 1 to 50 mole percent of Pb and 0.1 to 1.0 mole percent of Mg.

Japanese Patent Application Laid-Open No. 2-48464 discloses a semiconductive ceramic composition, in which a fraction of the Ba in BaTiO₃ is replaced with 0.001 to 0.1 atomic percent of Mg and 0.01 to 2.0 atomic percent of Ca, a fraction of Ba is replaced with 0.01 to 5.0 atomic percent of Pb and 0.01 to 20 atomic percent of Ca, or a fraction of Ba is replaced with 0.001 to 0.1 atomic percent of Mg, 0.01 to 5.0 atomic percent of Pb, and 0.01 to 2.0 atomic percent of Ca to reduce a change in resistance with temperature within an operational environment temperature range and to reduce specific resistance at ordinary temperatures.

Japanese Patent Application Laid-Open No. 2-48465 discloses a barium titanate-based semiconductive ceramic composition, in which a fraction of the Ba in BaTiO₃ is replaced with 0.001 to 0.1 atomic percent of Mg, a fraction of Ba is replaced with 0.01 to 5.0 atomic percent of Pb, or a fraction of Ba is replaced with 0.001 to 0.1 atomic percent of Mg and 0.01 to 5.0 atomic percent of Pb to reduce a change in resistance with temperature within an operational environment temperature range.

With miniaturization and high-density trends in recent electronic devices, miniaturization of positive coefficient thermistor devices composed of barium titanate-based semiconductive ceramic compositions used in the electronic devices has also progressed. However, miniaturization of positive coefficient thermistors causes deterioration of rush current resistance characteristics (flash withstand voltage characteristics); hence, no conventional positive coefficient thermistor meets commercial miniaturization requirements.

DISCLOSURE OF INVENTION

It is an object of the present invention to provide a barium titanate-based semiconductive ceramic composition having improved rush current resistance characteristics, thus facilitating miniaturization of positive coefficient thermistor devices.

The present invention has been completed to achieve such an object.

A barium titanate-based semiconductive ceramic composition in accordance with the present invention comprises a major component composed of barium titanate or a solid solution thereof, a semiconductivity-imparting agent, and an additive, wherein a fraction of the Ba in BaTiO₃ as the major component is replaced with 1 to 25 mole percent of Ca, 1 to 30 mole percent of Sr, and 1 to 50 mole percent of Pb, and wherein to 100 mole percent of the major component, the semiconductivity-imparting agent is added in an amount of 0.2 to 1.0 mole percent as a converted element content, and the additive comprises manganese oxide in an amount of 0.01 to 0.10 mole percent as a converted Mn content, silica in an amount of 0.5 to 5 mole percent as a converted SiO₂ content, and magnesium oxide in an amount of 0.028 to 0.093 mole percent as a converted Mg content.

In the barium titanate-based semiconductive ceramic composition in accordance with the present invention, the semiconductivity-imparting agent is preferably at least one element selected from the group consisting of Y, La, Ce, Nb, Bi, Sb, W, Th, Ta, Dy, Gd, Nd, and Sm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship between the current of a positive coefficient thermistor device and the time of measurement.

BEST MODE FOR CARRYING OUT THE INVENTION

The embodiments of the present invention will now be described.

The barium titanate-based semiconductive ceramic composition in accordance with the present invention contains a major component composed of barium titanate or a solid solution thereof, in which a fraction of Ba is replaced with Ca, Sr, and Pb in the above-described amounts (mole percent), a semiconductivity-imparting agent, and an additive. The additive comprises manganese oxide, silica and magnesium oxide in the above-described amounts (mole percent). Such a composition facilitates improvement in rush current resistance characteristics, and thus facilitates miniaturization of positive coefficient thermistors.

By the partial replacement of Ba with Pb, Ca and Sr and the addition of Mg, the rush current resistance characteristics can be significantly improved compared to the conventional cases using only one or two components among these components together with Mg.

When the total amount is 100 mole percent, the amount of the major component is the value after subtracting the total mole percent of the semiconductivity-imparting agent and the additive from 100 mole percent.

Various semiconductivity-imparting agents may be used without limitation in the present invention. Examples of the semiconductivity-imparting agents include Y, La, Ce, Nb, Bi, Sb, W, Th, Ta, Dy, Gd, Nd and Sm.

The present invention will now be described in more detail with reference to the following examples; however, the present invention is not limited to these examples.

As raw materials, BaCO₃, CaCO₃, Pb₃O₄, SrCO₃, and TiO₂ as the major components, Y₂O₃, La₂O₃, Er₂O₃, and Nd₂O₃ as the semiconductivity-imparting agents, and MnCO₃, SiO₂, and MgCO₃ as the additives were prepared. These raw materials were compounded and then wet-mixed to prepare semiconductive ceramic compositions having the formulations shown in Tables 1 to 4. The compositions were dehydrated, dried, and then calcined at 1,100 to 1,200° C. for 2 hours. The calcined compositions were pulverized, wet-mixed with binders, granulated, and then compacted under a compaction pressure of 1,000 kg/cm² to form disks. The resulting disks were fired at 1,300 to 1,400° C. to form disk semiconductor ceramics having a diameter of 11.5 mm and a thickness of 2.2 mm.

Ni—Ag layered electrodes including an electroless nickel plating layer (first layer) and a silver baking layer (second layer) are formed on the two faces of each semiconductive ceramic.

Each sample was subjected to resistance measurement at room temperature (25° C.), withstand voltage characteristic, Curie temperature, and rush current resistance characteristic (flash withstand voltage characteristic), and the results are shown in Tables 5 to 8.

Among these characteristics, the withstand voltage characteristic means the maximum applied voltage just before the sample is broken when a voltage applied to the sample is gradually increased. The rush current resistance characteristic means the maximum voltage (flash resistance voltage) not causing breakage of the semiconductive ceramic when an alternate rush voltage is applied to the sample. Samples marked with an *(asterisk) are outside the scope of the present invention.

TABLE 1 Semiconductivity- Major Components (mole percent) imparting agent Additives (mole percent) Sample BaTiO₃ CaTiO₃ SrTiO₃ PbTiO₃ (mole percent) Mn SiO₂ Mg *1 87 0 8 5 Y: 0.4 0.05 2 0 *2 87 0 8 5 Y: 0.4 0.05 2 0.0028 *3 87 0 8 5 Y: 0.4 0.05 2 0.093 *4 86 1 8 5 Y: 0.4 0.05 2 0 5 86 1 8 5 Y: 0.4 0.05 2 0.0028 6 86 1 8 5 Y: 0.4 0.05 2 0.093 *7 86 1 8 5 Y: 0.4 0.05 2 0.150 8 84 3 8 5 Y: 0.4 0.05 2 0.0028 9 84 3 8 5 Y: 0.4 0.05 2 0.093 *10 82 5 8 5 Y: 0.4 0.05 2 0.28 11 72 15 8 5 Y: 0.4 0.05 2 0 12 72 15 8 5 Y: 0.4 0.05 2 0.00028 *13 72 15 8 5 Y: 0.4 0.05 2 0.0028 14 72 15 8 5 Y: 0.4 0.05 2 0.028 *15 72 15 8 5 Y: 0.4 0.05 2 0.150 16 67 20 8 5 Y: 0.4 0.05 2 0.028 *17 62 25 8 5 Y: 0.4 0.05 2 0.00028 *18 62 25 8 5 Y: 0.4 0.05 2 0.028 *19 62 25 8 5 Y: 0.4 0.05 2 0.150 *20 57 30 8 5 Y: 0.4 0.05 2 0.028 *21 57 30 8 5 Y: 0.4 0.05 2 0.093 *22 80 15 0 5 Y: 0.4 0.05 2 0 23 80 15 0 5 Y: 0.4 0.05 2 0.093 24 79 15 1 5 Y: 0.4 0.05 2 0 25 79 15 1 5 Y: 0.4 0.05 2 0.0028 26 79 15 1 5 Y: 0.4 0.05 2 0.093 *27 78 15 2 5 Y: 0.4 0.05 2 0.0028 28 78 15 2 5 Y: 0.4 0.05 2 0.093 *29 70 15 10 5 Y: 0.4 0.05 2 0 *30 70 15 10 5 Y: 0.4 0.05 2 0.0028 31 70 15 10 5 Y: 0.4 0.05 2 0.150 *32 55 15 25 5 Y: 0.4 0.05 2 0 *33 55 15 25 5 Y: 0.4 0.05 2 0.093 34 50 15 30 5 Y: 0.4 0.05 2 0 35 50 15 30 5 Y: 0.4 0.05 2 0.00028 *36 50 15 30 5 Y: 0.4 0.05 2 0.0028 *37 50 15 30 5 Y: 0.4 0.05 2 0.028 *38 45 15 35 5 Y: 0.4 0.05 2 0 *39 45 15 35 5 Y: 0.4 0.05 2 0.0028

TABLE 2 Semiconductivity- Major Components (mole percent) imparting agent Additives (mole percent) Sample BaTiO₃ CaTiO₃ SrTiO₃ PbTiO₃ (mole percent) Mn SiO₂ Mg *40 45 15 35 5 Y: 0.4 0.05 2 0.093 *41 77 15 8 0 Y: 0.4 0.05 2 0 42 77 15 8 0 Y: 0.4 0.05 2 0.0028 43 76 15 8 1 Y: 0.4 0.05 2 0 44 76 15 8 1 Y: 0.4 0.05 2 0.0028 45 76 15 8 1 Y: 0.4 0.05 2 0.093 46 75 15 8 2 Y: 0.4 0.05 2 0.0028 47 75 15 8 2 Y: 0.4 0.05 2 0.093 48 67 15 8 10 Y: 0.4 0.05 2 0.028 49 57 15 8 20 Y: 0.4 0.05 2 0.028 50 47 15 8 30 Y: 0.4 0.05 2 0.0028 51 47 15 8 30 Y: 0.4 0.05 2 0.093 *52 42 15 8 35 Y: 0.4 0.05 2 0.028 53 37 15 8 40 Y: 0.4 0.05 2 0.093 *54 37 15 8 40 Y: 0.4 0.05 2 0.150 55 32 15 8 45 Y: 0.4 0.05 2 0.028 *56 27 15 8 50 Y: 0.4 0.05 2 0 *57 27 15 8 50 Y: 0.4 0.05 2 0.0028 *58 22 15 8 60 Y: 0.4 0.05 2 0 *59 22 15 8 60 Y: 0.4 0.05 2 0.0028 *60 22 15 8 60 Y: 0.4 0.05 2 0.093 *61 72 15 8 5 Y: 0.1 0.05 2 0 62 72 15 8 5 Y: 0.1 0.05 2 0.0028 63 72 15 8 5 Y: 0.2 0.05 2 0.00028 *64 72 15 8 5 Y: 0.2 0.05 2 0.0028 *65 72 15 8 5 Y: 0.2 0.05 2 0.028 66 72 15 8 5 Y: 0.2 0.05 2 0.150 67 72 15 8 5 Y: 0.3 0.05 2 0 *68 72 15 8 5 Y: 0.3 0.05 2 0.093 *69 72 15 8 5 Y: 0.8 0.05 2 0.093 70 72 15 8 5 Y: 1.0 0.05 2 0 71 72 15 8 5 Y: 1.0 0.05 2 0.00028 72 72 15 8 5 Y: 1.0 0.05 2 0.0028 *73 72 15 8 5 Y: 1.0 0.05 2 0.028 *74 72 15 8 5 Y: 1.0 0.05 2 0.093 *75 72 15 8 5 Y: 1.0 0.05 2 0.150 *76 72 15 8 5 Y: 1.2 0.05 2 0 *77 72 15 8 5 Y: 1.2 0.05 2 0.028 78 74 12 10 4 Er: 0.1 0.05 2 0.028 *79 74 12 10 4 Er: 0.2 0.05 2 0

TABLE 3 Semiconductivity- Major Components (mole percent) imparting agent Additives (mole percent) Sample BaTiO₃ CaTiO₃ SrTiO₃ PbTiO₃ (mole percent) Mn SiO₂ Mg *80 74 12 10 4 Er: 0.2 0.05 2 0.028 *81 74 12 10 4 Er: 0.2 0.05 2 0.150 82 74 12 10 4 Er: 0.4 0.05 2 0 83 74 12 10 4 Er: 0.4 0.05 2 0.00028 84 74 12 10 4 Er: 0.4 0.05 2 0.0028 *85 74 12 10 4 Er: 0.4 0.05 2 0.028 *86 74 12 10 4 Er: 0.4 0.05 2 0.093 *87 74 12 10 4 Er: 0.4 0.05 2 0.150 *88 74 12 10 4 Er: 0.4 0.05 2 0.200 89 74 12 10 4 Er: 0.4 0.05 2 0.280 90 74 12 10 4 Er: 1.0 0.05 2 0 *91 74 12 10 4 Er: 1.0 0.05 2 0.0028 *92 74 12 10 4 Er: 1.0 0.05 2 0.093 *93 74 12 10 4 Er: 1.0 0.05 2 0.150 *94 74 12 10 4 Er: 1.2 0.05 2 0 *95 74 12 10 4 Er: 1.2 0.05 2 0.028 *96 72 15 8 5 La: 0.1 0.05 2 0 97 72 15 8 5 La: 0.1 0.05 2 0.0028 98 72 15 8 5 La: 0.2 0.05 2 0 *99 72 15 8 5 La: 0.2 0.05 2 0.0028 *100 72 15 8 5 La: 0.2 0.05 2 0.093 101 72 15 8 5 La: 0.2 0.05 2 0.150 102 72 15 8 5 La: 0.5 0.05 2 0 *103 72 15 8 5 La: 0.5 0.05 2 0.0028 *104 72 15 8 5 La: 0.5 0.05 2 0.093 *105 72 15 8 5 La: 0.5 0.05 2 0.150 *106 72 15 8 5 La: 1.2 0.05 2 0 107 72 15 8 5 La: 1.2 0.05 2 0.028 108 72 15 8 5 Nd: 0.4 0.05 2 0 *109 72 15 8 5 Nd: 0.4 0.05 2 0.0028 *110 72 15 8 5 Nd: 0.4 0.05 2 0.093 *111 72 15 8 5 Nd: 0.4 0.05 2 0.150 *112 72 15 8 5 Y: 0.4 0.005 2 0 *113 72 15 8 5 Y: 0.4 0.005 2 0.0028 114 72 15 8 5 Y: 0.4 0.005 2 0.093 115 72 15 8 5 Y: 0.4 0.01 2 0 *116 72 15 8 5 Y: 0.4 0.01 2 0.0028 *117 72 15 8 5 Y: 0.4 0.01 2 0.093 *118 72 15 8 5 Y: 0.4 0.01 2 0.150 119 72 15 8 5 Y: 0.4 0.10 2 0

TABLE 4 Semiconductivity- Major Components (mole percent) imparting agent Additives (mole percent) Sample BaTiO₃ CaTiO₃ SrTiO₃ PbTiO₃ (mole percent) Mn SiO₂ Mg 120 72 15 8 5 Y: 0.4 0.10 2 0.00028 *121 72 15 8 5 Y: 0.4 0.10 2 0.0028 *122 72 15 8 5 Y: 0.4 0.10 2 0.093 *123 72 15 8 5 Y: 0.4 0.10 2 0.150 *124 72 15 8 5 Y: 0.4 0.12 2 0 *125 72 15 8 5 Y: 0.4 0.12 2 0.028 *126 72 15 8 5 Y: 0.4 0.12 0.2 0 *127 72 15 8 5 Y: 0.4 0.12 0.2 0.0028 *128 72 15 8 5 Y: 0.4 0.12 0.2 0.093 *129 72 15 8 5 Y: 0.4 0.12 0.5 0 *130 72 15 8 5 Y: 0.4 0.12 0.5 0.0028 *131 72 15 8 5 Y: 0.4 0.12 0.5 0.093 *132 72 15 8 5 Y: 0.4 0.12 0.5 0.150 *133 72 15 8 5 Y: 0.4 0.12 5 0 *134 72 15 8 5 Y: 0.4 0.12 5 0.00028 *135 72 15 8 5 Y: 0.4 0.12 5 0.0028 *136 72 15 8 5 Y: 0.4 0.12 5 0.028 *137 72 15 8 5 Y: 0.4 0.12 5 0.093 *138 72 15 8 5 Y: 0.4 0.12 5 0.150 *139 72 15 8 5 Y: 0.4 0.12 8 0 *140 72 15 8 5 Y: 0.4 0.12 8 0.028

TABLE 5 Rush current Resistance Withstand voltage Curie point resistance Sample (Ω) (V) (° C.) characteristic (V) *1 5.7 200 125 180 *2 5.1 180 125 180 *3 5.1 180 125 200 *4 5.8 200 125 225 5 5.4 200 125 315 6 5.2 200 125 315 *7 14.3 315 125 250 8 7.3 250 129 355 9 7.1 250 129 355 10 + ∞ — — — 11 7.7 355 127 250 12 9.6 355 127 250 *13 9.4 315 127 400 14 9.4 315 127 400 *15 22.6 355 127 315 16 10.8 400 127 315 *17 11.6 400 126 355 *18 11.0 400 126 450 *19 27.1 355 126 355 *20 88.5 450 124 315 *21 89.3 450 124 315 *22 7.1 250 136 150 23 6.1 250 136 150 24 8.6 250 136 200 25 8.2 250 136 315 26 7.9 250 136 315 *27 8.2 250 136 355 28 7.9 250 136 315 *29 10.8 450 135 355 *30 9.4 450 134 500 31 35.5 400 135 400 *32 12.0 500 56 355 *33 10.6 500 56 500 34 13.2 500 54 355 35 13.5 500 54 355 *36 11.8 450 53 500 *37 10.2 500 55 500 *38 19.0 630 55 400 *39 18.3 630 56 400

TABLE 6 Rush current Resistance Withstand voltage Curie point resistance Sample (Ω) (V) (° C.) characteristic (V) *40 18.5 630 54 400 *41 6.1 224 91 120 42 5.7 224 90 120 43 6.3 250 90 150 44 5.7 250 91 250 45 5.5 280 90 225 46 6.1 225 102 250 47 5.7 225 103 250 48 13.5 560 141 500 49 21.0 630 195 630 50 26.9 710 233 900 51 26.5 710 231 900 *52 34.2 800 239 1,000 53 39.5 800 258 1,000 *54 180.6 560 258 710 55 43.4 800 281 1,000 *56 82.5 500 298 900 *57 68.5 500 296 1,000 *58 491.9 — — — *59 517.5 — — — *60 505.7 — — — *61 7.1K — — — 62 7.1K — — — 63 18.1 560 123 500 *64 17.5 560 124 710 *65 17.3 560 124 630 66 43.4 500 123 500 67 16.7 500 122 450 *68 15.3 500 124 630 *69 20.4 560 123 710 70 22.2 630 123 500 71 21.6 630 123 500 72 19.2 560 122 630 *73 18.8 560 124 630 *74 18.3 560 124 630 *75 50.3 500 122 500 *76 1.3K — — — *77 1.2K — — — 78 5.7K — — — *79 13.2 630 126 450

TABLE 7 Rush current Resistance Withstand voltage Curie point resistance Sample (Ω) (V) (° C.) characteristic (V) *80 12.2 630 127 630 *81 57.5 500 127 400 82 11.4 630 127 400 83 12.0 630 128 400 84 9.8 560 127 630 *85 9.4 630 127 630 *86 10.0 560 128 710 *87 17.7 500 127 450 *88 65.8 560 127 500 89 + ∞ — — — 90 14.7 630 129 450 *9l 13.5 560 128 630 *92 13.0 560 129 710 *93 68.7 630 129 500 *94 842.0 — — — *95 803.8 — — — *96 5.7K — — — 97 5.1K — — — 98 9.2 500 124 450 *99 8.4 500 122 560 *100 8.2 500 125 560 101 31.2 500 124 450 102 11.8 630 124 450 *103 10.4 630 124 630 *104 10.8 630 124 580 *105 54.2 500 123 450 *106 777.3 — — — 107 815.5 — — — 108 9.4 560 125 315 *109 8.4 500 125 500 *110 8.2 500 124 500 *111 28.1 500 125 400 *112 16.5 355 120 180 *113 15.1 355 120 180 114 14.8 355 120 180 115 18.1 560 121 355 *116 16.9 560 123 500 *1l7 16.3 560 121 500 *118 89.1 450 121 450 119 21.0 630 122 500

TABLE 8 Rush current Resistance Withstand voltage Curie point resistance Sample (Ω) (V) (° C.) characteristic (V) 120 21.4 630 121 500 *121 19.6 630 121 800 *122 19.2 630 122 800 *123 109.2 500 121 560 *124 80.7 800 121 800 *125 75.6 800 121 800 *126 247.2 — 122 — *127 245.8 — 122 — *128 235.2 — 121 — *129 82.3 1,000 121 800 *130 71.5 1,000 120 800 *131 72.8 1,000 120 800 *132 437.6 800 121 800 *133 55.6 900 120 710 *134 56.3 900 119 710 *135 51.6 900 119 630 *136 50.3 900 119 630 *137 49.1 800 120 630 *138 217.1 800 119 630 *139 Melted — — — *140 Melted — — —

The reasons for the numerical limitations of the scope of the composition in accordance with the present invention will now be described.

In the major component comprising barium titanate or a solid solution thereof, a fraction of the Ba in BaTiO₃ is replaced with 1 to 25 mole percent of Ca, 1 to 30 mole percent of Sr, and 1 to 50 mole percent of Pb for the following reasons.

When the Ca content is less than 1 mole percent, the effects of the addition are insufficient and the rush current resistance characteristic is lower than the withstand voltage characteristic, as shown in Samples 1, 2 and 3.

On the other hand, a Ca content of higher than 25 mole percent causes a significant increase in resistance and rush current resistance characteristics lower than a withstand voltage characteristic, as shown in Samples 20 and 21.

When the Sr content is less than 1 mole percent, the rush current resistance characteristic is lower than the withstand voltage characteristic, as shown in Samples 22 and 23. In Samples 22 and 23, Sr is not added. It is confirmed that the rush current resistance characteristic is also lower than the withstand voltage characteristic due to insufficient addition when less than 1 mole percent of Sr is added.

On the other hand, a Sr content of higher than 30 mole percent causes a significant increase in resistance and rush current resistance characteristics lower than a withstand voltage characteristic, as shown in Samples 38, 39 and 40.

When the Pb content is less than 1 mole percent, the rush current resistance characteristic is lower than the withstand voltage characteristic, as shown in Samples 41 and 42. In samples 41 and 42, Pb is not added. It is confirmed that the rush current resistance characteristic is also lower than the withstand voltage characteristic due to the insufficient addition, when less than 1 mole percent of Pb is added.

On the other hand, when the Pb content is more than 50 mole percent, semiconductors are not formed, as shown in Samples 58, 59 and 60.

Next, the amount of the added semiconductivity-imparting agent is limited to 0.2 to 1 mole percent to 100 mole percent of the major component for the following reasons.

When the amount is less than 0.2 mole percent, no semiconductor is produced due to insufficient effects of the addition and the resistance is extraordinarily high, as shown in Samples 61, 62, 78, 96 and 97.

On the other hand, when the amount is more than 1.0 mole percent, resistance is extraordinarily high, resulting in deterioration of the withstand voltage and rush current resistance characteristics, as shown in Samples 76, 77, 94, 95, 106 and 107.

Next, the amount of manganese as the additive is limited to 0.01 to 0.10 mole percent (converted to Mn) with respect to 100 mole percent of the major component for the following reasons.

When the amount is less than 0.01 mole percent, change in resistance with temperature is small, and this is not practical due to insufficient effects of the addition, as shown in Samples 112, 113, and 114.

On the other hand, when the amount is more than 0.10 mole percent, resistance is extraordinarily high for practical use, as shown in Samples 124 to 140.

Next, the amount as a converted SiO₂ content of silica as the additive is limited to 0.5 to 5 mole percent with respect to 100 mole percent of the major component for the following reasons.

When the amount is less than 0.5 mole percent, the effects of the addition are insufficient and a change in specific resistance caused by a slight change in the conductor-imparting agent content cannot be sufficiently suppressed, as shown in Samples 126, 127 and 128.

On the other hand, when the amount is more than 0.10 mole percent, change in specific resistance cannot be sufficiently suppressed, as shown in Samples 139 and 140.

Next, the amount as a converted Mg content of magnesium oxide as the additive is limited to 0.0028 to 0.093 mole percent with respect to 100 mole percent of the major component for the following reasons.

When the amount is less than 0.0028 mole percent, no improvement in the characteristics is observed due to a trace amount of additive, as shown in Samples 1, 4, 11, 12, 17, 22, 24, 29, 32, 34, 35, 38, 41, 43, 56, 58, 61, 63, 67, 70, 71, 76, 79, 82, 83, 90, 94, 96, 98, 102, 106, 108, 112, 115, 119, 120, 124, 126, 129, 133, 134 and 139.

On the other hand, when the amount is higher than 0.093 mole percent, resistance is significantly increased due to excessive addition, as shown in Samples 7, 10, 15, 19, 31, 54, 66, 75, 81, 87, 88, 89, 93, 101, 105, 111, 118, 123, 132 and 138.

In Table 9, samples based on Example 3 in Japanese Patent Publication No. 62-43522 as Comparative Samples were subjected to measurements of flash withstand voltage characteristics as in the above method. Table 9 also shows the Curie point (Tc) and the specific resistance (ρ). The amounts in each composition are represented by mole percent.

TABLE 9 Electrical characteristics Major component (mole percent) Flash withstand Sample Ba Pb R SiO₂ Mn Mg Tc(° C.) ρ(Ωcm) voltage (V) 201 89.7 10 Y: 0.3 1 0.03 0 170 67 180 202 89.7 10 Y: 0.3 1 0.03 0.003 170 63 180 203 89.7 10 Y: 0.3 1 0.03 0.004 169 62 180 204 89.7 10 Y: 0.3 1 0.03 0.028 169 63 180 205 89.7 10 Y: 0.3 1 0.03 0.093 169 75 200 206 89.7 10 Y: 0.3 1 0.03 0.150 168 200  250 207 89.7 10 Y; 0.3 1 0.03 0.200 167 1.9 × 10³ —

According to these Comparative Samples, sufficient flash withstand voltage is not achieved in Ba—Pb-based barium titanate semiconductive ceramic compositions even when the amount as a converted Mg content of magnesium is 0.028 to 0.056 mole percent.

In Table 10, Ba—Pb—Sr—Ca-based samples having substantially the same ρ and Tc values as those in Table 9 were prepared and subjected to measurement of flash withstand voltage characteristics as in the above method.

TABLE 10 Electrical characteristics Major component (mole percent) Flash withstand Sample Ba Pb Sr Ca R SiO₂ Mn Mg Tc(° C.) ρ(Ωcm) voltage (V) 208 66.6 12 8 15 Y: 0.4 2 0.05 0 170 64 315 209 66.6 12 8 15 Y: 0.4 2 0.05 0.003 170 59 400 210 66.6 12 8 15 Y: 0.4 2 0.05 0.064 170 57 450 211 66.6 12 8 15 Y: 0.4 2 0.05 0.014 169 53 450 212 66.6 12 8 15 Y: 0.4 2 0.05 0.028 169 52 500 213 66.6 12 8 15 Y: 0.4 2 0.05 0.093 168 56 500 214 66.6 12 8 15 Y: 0.4 2 0.05 0.150 168 120 355 215 66.6 12 8 15 Y: 0.4 2 0.05 0.200 167 600 355

These Ba—Pb—Sr—Ca-based samples result in improvement in flash withstand voltage characteristics and greater improvement in the flash withstand voltage characteristics when Mg is added within a range of the present invention.

In Table 11, samples containing only Ba as the major component (Samples 216 and 217), Ba—Sr-based samples (Samples 218 and 219), Ba—Ca-based samples (Samples 220 and 221), Ba—Pb—Sr-based samples (Samples 222 and 223), Ba—Pb—Ca-based samples (Samples 224 and 225), Ba—Sr—Ca-based samples (Samples 226 and 227), Ba—Pb-based samples (Samples 228 and 229), and Ba—Pb—Sr—Ca-based samples (Samples 230 and 231) were prepared and subjected to measurement of the flash withstand voltage characteristic as in the above method.

TABLE 11 Electrical characteristics Major component (mole percent) Flash withstand Sample Ba Pb Sr Ca R SiO₂ Mn Mg Tc(° C.) ρ(Ωcm) voltage (V) 216 100 0 0 0 Y: 0.4 2 0.05 0 129 22 70 217 100 0 0 0 Y: 0.4 2 0.05 0.0028 129 21 70 218 92 0 8 0 Y: 0.4 2 0.05 0 100 20 120 219 92 0 8 0 Y: 0.4 2 0.05 0.0028 100 19 120 220 85 0 0 15 Y: 0.4 2 0.05 0 128 22 100 221 85 0 0 15 Y: 0.4 2 0.05 0.0028 128 19 70 222 87 5 8 0 Y: 0.4 2 0.05 0 131 35 180 223 87 5 8 0 Y: 0.4 2 0.05 0.0028 131 32 150 224 80 5 0 15 Y: 0.4 2 0.05 0 136 34 150 225 80 5 0 15 Y: 0.4 2 0.05 0.0028 136 36 150 226 77 0 8 15 Y: 0.4 2 0.05 0 91 29 120 227 77 0 8 15 Y: 0.4 2 0.05 0.0028 90 27 120 228 95 5 0 0 Y: 0.4 2 0.05 0 138 23 100 229 95 5 0 0 Y: 0.4 2 0.05 0.0028 138 21 100 230 72 5 8 15 Y: 0.4 2 0.05 0 127 36 250 231 2 5 8 15 Y: 0.4 2 0.05 0.0028 127 33 400

Barium titanate-based semiconductive ceramic compositions containing Ba—Pb—Sr—Ca major components result in satisfactory flash withstand voltage characteristic when the magnesium content as a converted Mg content is 0.028 mole percent.

Using samples selected at random from Tables 1 to 4, disk devices provided with Ni−Ag electrodes were prepared and subjected to measurement of current attenuation characteristic (P_(max)) and stabilized current characteristic, and the results are shown in Table 12.

Herein, the current attenuation characteristic (P_(max)) is the maximum value of the envelope variations P=I₁−I2 wherein I₁ is a peak value and I₂ is the adjacent peak value, and the stabilized current characteristic is a current flowing in the circuit at three minutes from the start of the measurement.

TABLE 12 P_(max) (A) Stabilized current (mA_(p-p)) Resistance Acceptance criterion: Acceptance criterona: Sample (Ω) <3.4 A <4.9 mA *70 22.2 3.5 2.4 *71 21.6 3.6 2.2 72 19.2 3.3 1.6 73 18.8 3.2 1.7 74 18.3 3.2 1.7 *75 50.3 3.0 9.3 *82 11.4 3.9 5.6 *53 12.0 4.1 5.6 84 9.8 3.4 4.8 85 9.4 3.3 4.8 86 10.0 3.4 4.9 *87 17.7 2.8 7.5 *88 65.8 2.6 8.6 *89 +∞ — — *115 18.1 3.8 2.7 116 16.9 3.4 2.3 117 16.3 3.4 2.2 *118 89.1 3.0 8.2

As shown in Table 12, samples containing the major components, the semiconductivity-imparting agent and the additives other than magnesium within the above-described range, and containing 0.0028 to 0.093 mole percent as a converted Mg content of magnesium, have superior current attenuation (P_(max)) and stabilized current characteristics.

Use of the barium titanate-based semiconductive ceramic composition in accordance with the present invention facilitates further miniaturization of thermistor devices because of further improvement in the rush current characteristic (flash withstand voltage characteristic).

Since superior current attenuation and stabilized current characteristics are also achieved, electrical reliability is further improved.

Industrial Applicability

As described above, the barium titanate-based semiconductive ceramic composition in accordance with the present invention is applicable to a wide variety of electronic devices, for example, positive coefficient thermistor devices. 

What is claimed is:
 1. A barium titanate-based semiconductive ceramic composition comprising a major component composed of barium titanate or a solid solution thereof, a semiconductivity-imparting agent, and an additive; wherein a fraction of the Ba in BaTiO₃ as the major component is replaced with 1 to 25 mole percent of Ca, 1 to 30 mole percent of Sr, and 1 to 50 mole percent of Pb; and wherein to 100 mole percent of the major component, the semiconductivity-imparting agent is added in an amount of 0.2 to 1.0 mole percent as a converted element content, and the additive comprises manganese oxide in an amount of 0.01 to 0.10 mole percent as a converted Mn content, silica in an amount of 0.5 to 5 mole percent as a converted SiO₂ content, and magnesium oxide in an amount of 0.028 to 0.093 mole percent as a converted Mg content.
 2. A barium titanate-based semiconductive ceramic composition according to claim 1, wherein the semiconductivity-imparting agent is at least one element selected from the group consisting of Y, La, Ce, Nb, Bi, Sb, W, Th, Ta, Dy, Gd, Nd, and Sm. 